This invention relates to a method and apparatus for transmitting data packets over a communications link, and in particular to transmitting the packets over a communications link which is subject to a high error rate, such as a wireless link.
Wireless communication remains one of the most popular commercial methods for providing access and trunk communication mediums. With the recent launch of Low Earth Orbit (LEO) satellites, there are now many applications within commercial and military environments that may use terrestrial or satellite-based wireless communication links. The problem with wireless communication is that the fundamental error rates are significantly greater than those experienced in wireline systems. Wireless systems must cope with a harsher signal propagation environment which is subject to noise, interference, fading and delay. This is further compounded by the restrictions on the power levels at which wireless systems can operate. Mobile handsets and orbiting satellites are restricted in their transmit power by battery life, and wireless systems are generally constrained by regulatory limits on transmit power.
Wireless communication suffers errors within the traffic and the distribution of errors is uneven. The error rate has an underlying random independent bit error rate, overlaid by a burst error rate. It is possible to protect against a known random independent bit error rate with relative ease. Burst errors are more complex because they will corrupt large sections of data, and burst errors are particularly common where the communications signal is being interfered by repetitive pulses over a wireless link, or errors extended through transmission devices, for example, scramblers or modems.
Considering trunk communication, each trunk link will be used to carry several different types of traffic; the two most common traffic types being generally known as voice and data. Voice data can contain errors and still be understood to a reasonable quality due to the brain""s ability to cope with noise. The key criteria of voice is that it cannot withstand large variations in delay. Data traffic, such as a file transfer, can tolerate extreme delays but cannot tolerate lost or corrupt information.
In general these two different types of traffic have been carried by different networks operating significantly different protocols. More recently Asynchronous Transfer Mode (ATM) has provided a common network protocol for these two traffic types. ATM has been designed to operate over low error rate trunk networks, which generally use reliable optical communication techniques, and assumes that the data traffic suffers a low error rate. It has therefore generally been considered unsuitable for wireless transmission.
The ATM protocol segments data into cells. Each cell contains 48 octets of user data and 5 octets of network information (header). The header of a cell contains virtual channel connection (VCC) information, i.e. address information which allows the network to route the cell. The VCC is made from a concatenation of the Virtual Path Identifier and the Virtual Channel Identifier. These two fields require 28-bits (3.5 octets) of information.
Some attempts have been made to adapt ATM for transmission over wireless systems.
U.S. Pat. No. 5,568,482 (Li et al., assigned to Yurie Systems Inc.) describes a low speed radio link system for ATM transport. An incoming stream of ATM cells intended for transmission over the radio link is segmented into a plurality of subframes, each subframe carrying a plurality of ATM cells and having additional framing bytes. One example uses nine subframes, each carrying five ATM cells. The structure of this protocol allows synchronisation to be more easily maintained under burst error conditions on the link.
U.S. Pat. No. 5,600,653 (Chitre at al., assigned to Comsat Corporation) describes a technique for improving ATM operation over a communications link with bursty bit errors. Interleaved cells are formed at the transmit end of the link by combining into each interleaved cell certain bits from each of plural different original ATM cells. Examples interleave the header parts of 40 different cells to generate interleaved cells. Interleaving cells together is a technique that requires all of the information to be received and interleaved before transmission, e.g. all the ATM cells required to fill an interleaving frame will be stored within a device before transmission can begin. This results in a delay in constructing the interleaved set of cells and requires processing and storage. This is a general problem with interleaving in this manner. For example, a 30-cell interleaving frame operating between ingress and egress interfaces at 1.92 Mbps will require 6.625 msec to create the frame and a further 6.625 msec to transmit the frame.
Another problem with ATM is the cell delineation process that is used to synchronise the receiver to the ATM cell boundary. It is known to use a state machine having three states to search for a correct header. However, if any one header suffers two bit errors then the receiving device will not be able to determine the correct destination for the cell and will delete the cell. Therefore, a 2-bit error in the cell header causes complete loss of the cell, i.e. a 384-bit data error. This will occur with a probability of 0.1 over a stream of cells since the header is approximately one tenth of the bandwidth. A paper by K. L. Li, J. H. Kim and Y. H. Low entitled xe2x80x9cAdapting ATM in low speed environmentsxe2x80x9d attempts to solve this problem by allocating multiple redundant addresses which effectively sets up multiple virtual circuits to the same destination. The most probable error patterns in the address field will change the original address to another address which can be uniquely identified as the original address.
There are other problems with the cell delineation process associated with the amount of time it requires to establish and lose synchronisation.
The present invention seeks to provide a more reliable method for transmitting data packets over a communications link, such as a wireless link.
According to a first aspect of the present invention there is provided a method of transmitting data packets over a communications link in an interleaved manner, the link comprising a series of time-division multiplexed (TDM) frames, each frame comprising a plurality of time slots, a time slot being shorter than a data packet, the method comprising:
receiving data packets for transmission over the link;
for each data packet, transmitting the data packet over a series of the TDM frames, one time slot in each of the series of frames being used to transmit a portion of that data packet.
By interleaving the packets, burst errors are distributed across several channels, each channel suffering a level that can be corrected by a higher level protocol. This allows the traffic to pass with limited, or no additional protection requirements. An advantage of this method is that a significant number of packets is not delayed to form an interleaving frame. The allocation of data packets to the time slots in a TDM frame provides a suitable interleaving effect. The following description refers to this improved resilience to errors as link hardening.
Preferably each data packet is transmitted in the same time slot in each of the series of frames.
Preferably a received data packet is transmitted as soon as there is a free time slot in a frame, such that different ones of the data packets begin in different TDM frames. This has the advantages of not needing to wait for a batch of received cells before performing interleaving, as in the prior art, since the TDM structure provides the interleaving effect. This also minimises storage requirements at the transmitting entity.
The VPI and VCI fields in an ATM cell header require 28 bits (3.5 octets) of information. However, these 228=268 million addresses will not be used at the same time on one link. Therefore bandwidth can be gained on the communications link by sending a header that supports fewer addresses. An 8-bit address field permits 256 different addresses. This header is associated at both ends of the link such that the original header can be removed, the packet is associated with one of the 256 available channels, and the original header is reconstructed at the far end. By doing this some bandwidth is gained which can be used to improve the quality of the link. A shortened header is less likely to suffer an error, and the additional bandwidth that is gained by using a shortened header can be used to increase error protection for the header. Other numbers of bits can be used for the shortened header depending on the number of addresses that are needed for the link and the level of bandwidth saving that is required.
Preferably the modified header is split into a plurality of parts which are distributed across the length of the transmitted packet. This has an advantage that the header is more resilient to burst errors, because a burst will need to have a longer duration if it is to corrupt all of the distributed sections of the header.
Preferably error protection is included for the modified header and the error protection is distributed across the length of the transmitted packet. The error protection can comprise error detection and error correction. Preferably the header error protection is spaced, within the transmitted packet, from the other parts of the modified header by payload parts of the packet.
Preferably the transmitted packet includes synchronisation information to allow a receiving entity at a receiving end of the communications link to determine boundaries between transmitted packets on the link. This has the advantage of allowing the receiving entity to determine if a cell is present on the link, without using the packet address. Preferably the synchronisation information is distributed across the length of the transmitted packet.
Preferably the frames include synchronisation information, and the position of the synchronisation information is varied within the frames. If the synchronisation information is allocated to a time slot that should be carrying a portion of a data packet, that portion of the data packet is moved to occupy a predetermined alternative time slot within the frame. This alternative slot can be the first or last time slot in each frame, or some other position which is known by both ends of the link.
A further aspect of the invention provides apparatus to implement this method.
While the described method is particularly suited to use on a wireless communications link which is subject to high error rates, it will be appreciated that the method can also be used on other forms of communication link, such as wired links.
The following description refers to carrying ATM cell-based traffic over a communications link. However, it will be appreciated that the invention is not so limited, and encompasses other packet, cell or frame based transport mechanisms.
Preferred features may be combined as appropriate, and may be combined with any of the aspects of the invention, as would be apparent to a person skilled in the art.